1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) utilizing the electro-optical anisotropy of liquid crystal, and in particular, to an LCD for achieving a high aperture ratio and a wide viewing-angle.
2. Description of the Prior Art
The LCD is advantageous in that it is small and light-weight, and has low power consumption. It is thus put into practical use in such fields as in OA and AV equipment. In particular, the active matrix type which uses a thin film transistor (TFT) for the switching element can, in principle, v perform static drive of 100% duty ratio in a multiplexed manner, and is used for large screen, animation displays.
FIGS. 1 and 2 show the structure of a unit pixel for a conventional LCD; FIG. 1 is a plane view, and FIG. 2 is a cross-sectional view along the line Fxe2x80x94F in FIG. 1. A substrate 200 made of glass, etc., is provided, and a gate electrode 201 made of Cr, etc., and a gate line 202 made of Cr, etc., are formed thereon. Gate line 202 integrally connects a row of gate electrodes 201 aligned in the same row direction. Covering them, a gate insulation film 203 made of Si3N4, etc., is formed. On gate insulation film 203, there is formed an island-like amorphous silicon (a-Si) layer 204 in a region corresponding to gate electrode 201. The a-Si layer 204 will act as an operating layer for the TFT. Amorphous silicon regions doped with impurities, are formed at both ends of a-Si layer 204 creating (N+a-Si) layers 206 to act as contact layers. Between a-Si layer 204 and (N+a-Si) layers 206, an etching stopper 205 made of Si3N4 is formed as required for structural reasons. Further, on (N+a-Si) layers 206, there are respectively disposed a source electrode 207 and a drain electrode 208 both made of material having a high-melting point, such as Al/Si, etc. In regions other than a TFT region on gate insulation film 203, a pixel electrode 210 is formed, made of Indium Tin Oxide (ITO) which is transparent and conductive. Further, a drain line 209 is also provided for integrally connecting a column of drain electrodes 208 aligned in the same column direction. Covering all of the components mentioned above, an alignment layer 211 made of a polymer film, such as polyamide, is formed. Alignment layer 211 is subjected to predetermined rubbing processing for controlling the initial orientation of liquid crystal molecules. On alignment layer 211, a liquid crystal layer 230 is formed, on which another glass substrate 220 is disposed opposing substrate 200. A common electrode 211 made of ITO is formed on the entire surface of glass substrate 220, opposing substrate 200. Common electrode 221 is covered by an alignment layer 222 made of polyamide, etc., which is subjected to rubbing processing.
Liquid crystal is a nematic phase having, for instance, positive anisotropy of dielectric constant. When it is used for an LCD, a twist nematic (TN) mode is employed in which orientation vectors of liquid crystal molecules are twisted by 90 degrees between the top and bottom substrates 200, 220. A polarizing plate (not shown) is generally provided outside each substrates 200/220 such that, in the TN mode, a polarizing axis thereof matches a rubbing direction of alignment layer 211/222 on corresponding substrate 200/220. Thus, when no voltage is applied, linearly polarized light incoming through one of the polarizing plates proceeds within liquid crystal layer 230 while revolving along the twisted orientation of the liquid crystal molecules, until it comes out from the other polarizing plate. The LCD then displays white. On the other hand, a predetermined voltage is applied between pixel electrode 210 and common electrode 221, and an electric field is formed in liquid crystal layer 230, so that liquid crystal molecules change their orientation due to their dielectric constant anisotropy such that their long axes become parallel to the electric field. As a result, the twisted orientation of the liquid crystal molecules is destroyed, and incoming linearly polarized light is thus forced to stop revolving in liquid crystal layer 230. Then, only a reduced amount of light comes out from the other polarizing plate, resulting in a gradual change of a displayed color to black. The above mode in which an LCD displays white with no voltage applied and black with voltage applied is referred to as a normally-white mode, which is mainly employed for TN cells.
Another example is a DAP (deformation of vertically aligned phases)-type LCD which uses a nematic phase having a negative anisotropy of dielectric constant for an LCD, and a vertical alignment layer for DAP orientation films 211, 222. A DAP-type LCD, which is one example of those employing electrically controlled birefringence (ECB), utilizes a difference in a refractive index between a long axis and a short axis of liquid crystal molecules, i.e., birefringence, for controlling transmission and displayed colors. For this type, a polarizing plate is formed in a crossed Nicols arrangement outside each of substrates 200, 220. When voltage is applied, linearly polarized light introduced via one of the polarizing plates is converted into elliptically polarized light via birefringence in liquid crystal layer 230. The retardation of this elliptically polarized light, i.e., the difference in phase speed between ordinary and extraordinary ray, is controlled according to the strength of electric fields generated in liquid crystal layer 230, wherein the strength of the electric fields are determined depending on a voltage applied to liquid crystal layer 23. Then, colored light as desired will come out from the other polarizing plate on the LCD at a desired transmission according to the controlled retardation amount. As described above, an LCD obtains a desired transmission or displays desired hues by controlling light revolution or birefringence in liquid crystal. This controlling is effected by applying a desired voltage to liquid crystal which is sandwiched by a pair of substrates having predetermined electrodes formed thereon. In other words, in a TN mode, the strength of transmitted light can be controlled by controlling a retardation amount by changing the orientation of liquid crystal molecules. In an ECB mode, hue separation is also achieved by controlling the strength of transmitted light, which depends on wavelength. A retardation amount depends on an angle formed by a long axis of a liquid crystal molecule and the direction of an electric field generated in the liquid crystal. However, even if this angle is primarily controlled by adjusting electric field strength, a relative retardation amount will vary depending on an angle at which an observer views the LCD, i.e., a viewing angle. As viewing angle varies, the strength or the hues of transmitted light also changes. This is a problem of view angle dependency of an LCD.
This invention has been conceived to overcome the above problems and aims to provide a liquid crystal display for achieving a wider viewing angle through appropriate control of the orientation of liquid crystal molecules.
In order to achieve the above object, according to one aspect of the present invention, there is provided a liquid crystal display comprising a first substrate including a plurality of pixel electrodes arranged in a matrix for driving a liquid crystal; a thin film transistor having a drain electrode, a gate electrode, a semiconductor layer, and a source electrode connected to corresponding one of the plurality of pixel electrodes; a gate line connected to the gate electrode; and a drain line connected to the drain electrode; a second substrate disposed opposing the first substrate having a liquid crystal layer in between, wherein the plurality of pixel electrodes are formed on an inter-layer insulation film which has been formed covering the thin film transistor, the gate line, and the drain line; a common electrode for driving liquid crystal is formed on the second substrate; and an orientation control window of a predetermined pattern is formed to have no common electrodes formed in a region opposing each one of the corresponding plurality of pixel electrodes.
With this arrangement, a liquid crystal layer is situated away from a TFT and associated electrode lines. This distance between the former and the latter results in protecting a weak electric field around the orientation control window and an electric field sloped around the edges of a pixel electrode against the influence of electric fields generated by the TFT and lines. Then, the orientation of liquid crystal molecules can be controlled secondarily in a preferable manner through these sloped electric fields. To be more specific, variation in a retardation amount for the entire pixel can be suppressed in a structure which has been designed such that increase or decrease of a retardation amount at respective points inside a pixel is offset despite varied viewing angle. This structure can be achieved, utilizing the fact that the horizontal orientation of liquid crystal molecules in a pixel is determined according to the shape of the orientation control window. Moreover, since the above arrangement is effective in keeping the diagonal electric fields free from disturbance, it is possible to perform effective controls around the edges of a pixel electrode and an orientation control window. Therefore, preferable pixel dividing and a wider angle of visibility are realized.
In particular, an inter-layer insulation film is formed to have a thickness xcex1 of 0.5 xcexcm, preferably 1 xcexcm or more. Alternatively, the thickness xcex1 is defined to be at least half a distance "khgr" between two adjacent pixel electrodes.
With an inter-layer insulation film of the above thickness, a TFT and associated electrode lines do not affect a liquid crystal layer through electric fields. Then, the orientation of liquid crystal molecules can be effectively secondarily controlled through sloped electric fields generated in the liquid crystal layer around the edges of an orientation control window and a pixel electrode.
In one aspect, a pixel electrode is formed in a region defined by the gate and drain lines, overlapping, via the inter-layer insulation film, with the gate line and/or the drain line.
That is, separated by an inter-layer insulation film, a pixel electrode is situated in a different layer from that of gate and drain lines, having a sufficient interval between them. This allows provision of a pixel electrode overlapping with the gate and drain lines so that a larger display region and an increased aperture ratio can be secured.
In another aspect, at least a part of a TFT and gate and drain lines are disposed below a pixel electrode having an inter-layer insulation film in between them.
With this arrangement, the orientation of liquid crystal molecules can be kept free from disturbance due to the influence of electric fields caused by the TFT and lines on the liquid crystal.
In another aspect, a TFT and gate and drain lines are situated, being projected from a region corresponding to the bottom of a pixel electrode by an extent y, wherein half of the extent y is equal to or less than a thickness xcex1 of the inter-layer insulation film, or half of an interval "khgr" between two adjacent pixel electrodes.
The projected TFT and associated lines as above do not cause any difference in the effect in eliminating the influence of the electric fields caused by the TFT and lines on the liquid crystal.
In another aspect, an orientation control window is formed in a region opposite to a pixel electrode, for instance, along the diagonal line of the pixel electrode. Alternatively, it is shaped substantially like the letter X such that its crossing point falls on the opposing region in corresponding to around the center of the pixel electrode.
With either arrangement, when a voltage is applied between a common electrode and respective pixel electrodes, electric fields are generated in the sloped direction around the edges of the orientation control window and the pixel electrode without receiving adverse influence from electric fields which have been generated by the electrodes and lines situated below. As a result, liquid crystal molecules can be easily controlled to have appropriate orientation.
In another aspect, an orientation control window is formed including a linear part extending straight and substantially in parallel to any of the edges of a pixel electrode, and branch parts extending continuously from both ends of the linear part toward respective corners of the pixel electrode, wherein the linear part is formed on a region opposing a region around the center of the pixel electrode. With this arrangement, the orientation of liquid crystal molecules can be controlled to be in a further appropriate condition.